Collection: Move vs. Link
As you already discovered an object can be placed into any arbitrary number of collections, even on different scenes.
Moving or linking an object is really the same underlying action in relation to collection ownership. There is no &quot;real&quot; and &quot;instanced&quot; duplicate of an object that belongs to more than one collection, and the object truly is in multiple simultaneously with the same degree of importance.
So what is the different between &quot;Linking&quot; and &quot;Moving&quot; to a collection? If an object is moved to a collection it is removed from all others and placed in that one exclusively.
Say you have a car in your scene currently sitting in the Street collection, and you Move it to the Vehicles collection. Your car will now sit exclusively in the Vehicles collection.
Linking an object to a collections adds it to that collection without removing it from any others it curreclty sits in.
Using the same car example, if you have a car in your scene currently sitting in the Street collection, and you Link it to the Vehicles collection, your car will now sit both in the Vehicles collection and the Street. Both instances are real both are valid.
So why can you Unlink an object that has been moved to a collection? Likewise unlinking an object from a collection will remove it from that collection but not from others.
If the object is only in a single collection and you unlink it, it is removed from the current scene. If the same object is not used in any other scene it becomes Orphaned Data and can be purged out of the current file.